Four Torah commandments deal with the prohibition against ḥametz on Pesaḥ: three negative and one positive.
The first prohibition is to refrain from eating ḥametz, as it is written, “And ḥametz shall not be eaten” (Shemot 13:3). Our Sages taught that the prohibition against eating ḥametz on Pesaḥ includes not deriving any kind of benefit from the ḥametz. The Torah further states: “You shall not eat any leavened product (maḥmetzet)” (Shemot 12:20). Our Sages concluded from this verse that not only something that had fermented on its own is prohibited, but even food that had been leavened by some external agent may not be eaten on Pesaḥ. It must be noted that the Torah was particularly stringent concerning the prohibition against eating ḥametz. Almost all of the Torah’s food prohibitions are punishable by lashes, while eating ḥametz on Pesaḥ is punishable by karet (extirpation), as it is written, “whoever eats ḥametz from the first day until the seventh day, that soul shall be cut off from Israel” (Shemot 12:15).
The second prohibition is that no ḥametz may be found in our possession, as it is written, “Seven days there shall be no se’or found in your homes” (Shemot 12:19). Se’or is the “starter” or leavening agent used to make dough ferment. This verse means not only that se’or is forbidden, but also that no ḥametz may be found in our possession. This prohibition is often called bal yimatzei.
The third prohibition is that no ḥametz may be seen in our possession, as it is written: “Matzot shall be eaten seven days; and no ḥametz of yours shall be seen, and no se’or of yours shall be seen within all your borders” (Shemot 13:7). One violates the second prohibition (bal yimatzei) and this third prohibition (called bal yera’eh) only if one has in his possession on Pesaḥ at least one olive’s bulk (kezayit) of ḥametz. If the volume of the ḥametz that remained in one’s possession was less than a kezayit, he does not violate bal yera’eh and bal yimatzei on account of that ḥametz.
The fourth mitzva – a positive commandment – is to eliminate ḥametz and se’or in advance of Pesaḥ, as it is written, “Seven days you shall eat matzot; however, on the first day you shall remove the se’or from your houses” (Shemot 12:15).